1. Field of the Invention
This invention relates to steel tooth rolling cutter drill bits utilized for drilling boreholes in the earth for the minerals mining industry.
2. Setting of the Invention
Hardmetal inlays or overlays are employed in rock drilling bits as wear and deformation resistant cutting edges and faying surfaces. These typically comprise composite structures of hard particles in a more ductile metal matrix. The hard particles may be metal carbides, such as either the cast WC/W2C eutectic or monocrystalline WC, or may themselves comprise a finer cemented carbide composite material. Often, a combination of hard particle types is incorporated in the materials design, and particle size distribution is controlled to attain desired performance under rock drilling conditions, such as disclosed in U.S. Pat. Nos. 3,800,891; 4,726,432; and 4,836,307. The matrix of these hardmetal systems may be iron, nickel, or copper based, but whether formed by weld deposition, brazing, plasma spraying, or infiltration, the matrix microstructure is invariably a solidification product. During fabrication, the hard phase(s) remain entirely or at least partially solid, but the matrix phase(s) grow from a melt during cooling and thus are limited by thermodynamic, kinetic, and heat transport constraints to narrow ranges of morphology, constituency and crystal structure.
The strongest commonly employed hardmetals in rolling cutter rock bit cutting structures are made by weld application of sintered tungsten carbide based tube metals or composite rods utilizing iron based matrix systems. These hardmetal deposits undergo heat treatment prior to use, resulting in matrices which are essentially alloy steels by chemistry. Microstructurally the matrix is comprised of tempered martensite with minor amounts of carbide precipitates and retained austenite. Any austenite in the microstructure occupies the internecine spaces between martensite lathes or plates. The intrinsic difficulty in the control of heat input during weld deposition of hardfacing overlays results in matrix variation due to alloying effects arising from melt incorporation of sintered carbide hard phase constituents as well as substrate material. Partial melting of cemented carbide constituents resulting in "blurring" of the hard phase boundaries and the incorporation of cobalt and WC particles into the matrix. As a practical matter, process control is challenged to maintain "primary" hardmetal microstructural characteristics such as constituency and volume fraction relationships of hard phases. Secondary characteristics such as matrix microstructure are derivative and cannot be readily regulated.
The advent of rapid, solid state densification powder metallurgy (RSSDPM) processing of composite structures has enabled the fabrication of hardmetal inlays/overlays which potentially include a range of compositions and microstructures not attainable by solidification. In addition, RSSDPM processing also provides more precise control of microstructural features than that attainable with fused overlays. Such fabrication methodologies for rock bits are disclosed in U.S. Pat. Nos. 4,554,130; 4,592,252; and 4,630,692. Also disclosed therein and also in U.S. Pat. No. 4,562,892 are some preferred embodiments of drill bits with wear resistant hardmetal overlays which exploit the flexibility and control afforded by RSSDPM. Although many unique hardmetal formulations are made possible by RSSDPM, most will not be useful as rock bit hardmetal inlays because they lack the necessary balance of wear resistance, strength, and toughness. Unique RSSDPM composites can exhibit similarly unique failure progressions which disadvantage them for use in drilling service. For example, a RSSDPM "clone" of a conventional weld applied hardmetal made from 60 wt % cemented carbide pellets (30/40 mesh WC-7%Co), and 40 wt % 4620 steel powder, was found to have lower wear resistance than expected due to selective hard phase pullout caused by shear localization cracking in the matrix.
The presence of sharpened interfaces combined with the formation of ferrite "halos" around carbide pellets lead to deformation instability under high strain conditions. Even though the primary characteristics normally used to evaluate hardmetal (volume fractions, pellet hardness, matrix hardness, and porosity) were superior to conventional material, the RSSDPM clone exhibited an unexpected weakness. In another experiment, a RSSDPM formulation similar to the above example but adding a few percent of free (7 micrometer) WC powder was intended to mimic the precipitation induced dispersion strengthening of matrix in conventional hardmetal.
However, rapid surface diffusion in the powder preform prior to hot pressing caused transformation of the free WC to brittle eta type carbide in the final composite. In this case, an unexpected reaction led to compromise of the intended matrix strengthening mechanism.
The potential benefits of RSSDPM hardmetal inlays are thickness and microstructural uniformity, low defect and porosity levels, and stability of hard phases/hardness retention. In order to realize these benefits, special chemistry and microstructural design of the hardmetal matrix are required to provide appropriate deformation characteristics under high unit loads experienced at tooth crests.